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A Memorable Experiment on Heavy Fermions 17 April 1986  

by Michael Springford

Having missed the deadline for applications for a Lectureship in Physics at the shiny new University of Sussex in 1963, I was offered a Temporary Lectureship by the then Physics Chairman, Professor Ken Smith, which I took up on 1 Oct 1964. The first few years were unsettling as the mathematics and physics (MAPS) building was incomplete and academic life began in a temporary Nissen hut. Such buildings were not designed with acoustic insulation in mind, so that telephone and other conversations as well as tutorial sessions penetrated clearly into the adjacent rooms. But around 1966 the brick building was finally commissioned and I could begin to create a low temperature research laboratory for solid state physics.

My initial plan was to focus on the use of Landau Quantum Oscillations (and in particular the de Haas-van Alphen effect) to investigate the electronic structure of ordered metallic alloys, but this soon evolved into a rather different study to measure the variation of conduction electron lifetimes over the Fermi Surface. Having obtained a research grant (from the then SRC) for a powerful superconducting magnet for such work, the informality of University life at the time is well illustrated by the events surrounding its delivery. An employee of the manufacturer arrived unannounced, carried the magnet into the laboratory and asked for a cheque for a sum of around £10,000. Remarkably, I had no problem in obtaining it at a moment’s notice from the Finance Office!

Electron lifetime studies continued for several years but during the late 1970’s and early 1980’s researchers around the world reported the discovery of what soon came to be called ‘heavy-fermion systems’. On the basis of transport and transport measurements, these novel materials appeared to be metals in which conduction electrons (fermion quasiparticles) appeared to have masses as much as 1000 times greater than the free electron mass. Such numbers presented a serious challenge to the prevailing theoretical models. Seen in terms of my own experiments the implications were severe, for they implied that to study such ‘heavy electrons’, if indeed they existed, would require temperatures 1000 times lower than the already low temperature of 1K (i.e. 1 degree Kelvin above the absolute zero of temperature) that I was used to. This new temperature range could be achieved using a dilution refrigerator, but this was expensive, complex and notoriously difficult to use, as I had witnessed in other low temperature laboratories, including those of Professor Brewer in Sussex at the time. Furthermore it was signally ill suited to my experiments that, in addition to a low temperature, required an intense magnetic field and ready interchange of samples. Reflecting on this problem, I formed an idea for a new piece of equipment in which an experimental sample could be Top-Loaded directly into the Mixing chamber of a dilution refrigerator, a TLM dilution refrigerator. The development work was undertaken by the Oxford Instrument Company and I was able to place the order with them in June 1983, and had a fully working system in 1985. It changed forever the dilution refrigerator from being an exotic beast, employed mainly in helium research, to being a routine piece of equipment found in many solid state physics laboratories around the world.

The stage was now set for what became, for me, one of my most cherished memories as an experimentalist. The 17 April 1986 was already a very long day by 5pm. I had come in around 7am to help my research assistant, a Dutchman Dr Paul Reinders, to begin to prepare the new apparatus for its first serious experiment, before giving two undergraduate lectures in the morning and attending a long meeting during the afternoon. The installation of the new equipment had occupied us for the past several months, for its overall size had necessitated the removal of a large section of concrete ceiling, so connecting our laboratory to the room above, and the addition of a spiral staircase to give rapid access to all parts of the apparatus. It is ironic that such a large piece of equipment was required to perform an experiment on such a tiny sample, but this carefully prepared sample, whose size was less than 1mm, had been mounted in its measurement probe and inserted into the apparatus during the previous evening. Paul’s task was now to begin to cool it to a temperature of around 10 millikelvin, which for technical reasons needed to be done slowly whilst carefully monitoring its progress along the way. At around 9pm things appeared to be progressing well and we took a short break for supper, returning in the dark to a laboratory locked and devoid of people.

Working near the maximum design limit of a superconducting magnet is a nervous activity for they are intrinsically unstable instruments and can switch out of their superconducting state without warning, a phenomenon known to practitioners as a ‘quench’. While in a well designed system this is not dangerous, it can be frightening as a large volume of cryogenic fluid (liquid helium in this case) is rapidly vaporised to create a high pressure, rendering a quench explosive in character (such as was featured in the early days of the LHC!).

Installing the new TLM cryomagnetic system (referred to above) in 1985,                                                                                                      as featured on the Silver Jubilee edition of the University of Sussex                                                    Annual Report 1986, the picture shows Mike Springford and Paul Reinders.

Installing the new TLM cryomagnetic system (referred to above) in 1985,
as featured on the Silver Jubilee edition of the University of Sussex
Annual Report 1986, the picture shows Mike Springford and Paul Reinders.

The next few hours were spent testing and checking each part of the experiment and then finally commencing to energise the magnet. The individual instruments had been extensively tested during the preceding weeks and had successfully played their tunes for us, but this was the first occasion on which the whole orchestra had come together. Our aim was to cool the sample to the lowest temperature (10mK) in as high a magnetic field as possible (15Tesla). We achieved this at around 1a.m. Then, on changing the magnetic field, we saw for the first time the characteristic quantum oscillations in a heavy fermion metal. It was an intensely thrilling moment. But the best was to come for, by repeating the experiment at different low temperatures, we were then able to measure the effective mass of the electrons. Several hours later we had the result that we hoped for, the group of electrons that we were studying were heavier than free electrons by a factor of around 100; we had confirmed the existence of heavy electrons rather directly and in the following years even heavier ones were discovered. The result was enthusiastically received at the International Conference, ICAREA, that year in Grenoble and it initiated a renaissance in heavy fermion studies in many laboratories around the world.